1. Field of the Invention
The present invention relates to a wireless power supply technique.
2. Description of the Related Art
In recent years, wireless (contactless) power transmission has been receiving attention as a power supply technique for electronic devices such as cellular phone terminals, laptop computers, etc., or for electric vehicles. Wireless power transmission can be classified into three principal methods using an electromagnetic induction, an electromagnetic wave reception, and an electric field/magnetic field resonance.
The electromagnetic induction method is employed to supply electric power at a short range (several cm or less), which enables electric power of several hundred watts to be transmitted in a band that is equal to or lower than several hundred kHz. The power use efficiency thereof is on the order of 60% to 98%. In a case in which electric power is to be supplied over a relatively long range of several meters or more, the electromagnetic wave reception method is employed. The electromagnetic wave reception method allows electric power of several watts or less to be transmitted in a band between medium waves and microwaves. However, the power use efficiency thereof is small. The electric field/magnetic field resonance method has been receiving attention as a method for supplying electric power with relatively high efficiency at a middle range on the order of several meters. Rerated techniques have been disclosed in “A. Karalis, J. D. Joannopoulos, M. Soljacic, “Efficient wireless non-radiative mid-range energy transfer” ANNALS of PHYSICS Vol. 323, January 2008, pp. 34-48”, for example.
FIG. 1 is a diagram showing a wireless power supply system according to a comparison technique. The wireless power supply system 1r includes a wireless power transmitting apparatus 2r and a wireless power receiving apparatus 4r. The wireless power transmitting apparatus 2r includes a transmission coil LTX, a resonance capacitor CTX, and an AC power supply 10r. The wireless power receiving apparatus 4r includes a reception coil LRX, a resonance capacitor CRX, and a load 70.
In such a wireless power supply system 1r, in order to efficiently supply power, there is a need to satisfy the condition for resonance in the wireless power transmitting apparatus 2r and the wireless power receiving apparatus 4r. 
In such a wireless power supply system 1r, the transmission coil LTX and the reception coil LRX are each configured as a solenoid coil, a spiral coil (loop coil), or the like. The magnetic field (electric field or electromagnetic field) generated by such a coil is symmetrical with respect to the coil face. However, in many applications, the power receiving apparatus 4r is arranged on only one side of the coil face. In this case, in order to suppress interference with other electronic devices, such an arrangement is preferably configured to damp the magnetic field on the side on which the wireless power receiving apparatus 4r is not arranged. Such a damping function is important from the viewpoint of protecting the human body. As an approach for damping the magnetic field on the other side of the coil face, an arrangement is conceivable in which a magnetic sheet is arranged on one side of the coil so as to provide a magnetic bypass effect. However, an electric power signal employed in wireless power transmission has a frequency on the order of several hundreds of kHz to 10 MHz. There is no known magnetic material having sufficient magnetic permeability and small energy loss in this frequency band.